Activision and Infinity Ward are set to reveal Call of Duty: Modern Warfare II's multiplayer mode along with other details at the Next showcase event on September 15th. They also announced that the early access beta will start rolling out on September 16 and 17 on PlayStation consoles and arrive in an open crossplay beta to all consoles and PCs starting September 24th.
As shown below, you'll need to pre-order to get the open and early access beta releases, with PlayStation owners getting first dibs. After the early access, PlayStation 4 and 5 users will get the open beta from September 18-20, and then Xbox and PC will get early access (crossplay beta) from September 22-23, with PlayStation getting the open crossplay beta on the same dates. Finally, the open beta (crossplay) will be available on all platforms from September 24-26.
Along with the multiplayer mode, Infinity Ward said it would show "the imminent future of [what] Call of Duty will look like, including many more details regarding Modern Warfare II, information on the next Call of Duty: Warzone, and more on the mobile version of Call of Duty: Warzone (also known as Project Aurora)." It also promised you'd see YouTubers streamers playing the games in real time, along with surprise information.
If you haven't pre-ordered, you may still be able to get a beta code for early access from various streamers and YouTubers. Modern Warfare II is the successor to 2019's Call of Duty: Modern Warfare, based on the original Modern Warfare subseries launched in 2007. The new title is set to arrive on October 28th and cost $70 across all platforms.